Diltiazem has the following chemical formula: ##STR4## As can be seen, diltiazem has two asymmetric carbons at the starred positions. Because it has two asymmetric carbons, diltiazem is one of four possible stereoisomers.
Diltiazem is the most useful of the four stereoisomers, and as pictured above, it can be seen that it has the (2S, 3S) configuration at the two asymmetric positions.
Since diltiazem is one of four stereoisomers, in a nonstereospecific syntheses, the desired (2S, 3S) isomer can be obtained in at most a 25% yield.
Known processes for preparing diltiazem which fix the starred positions (the 2- and 3-positions) in the cis relative configuration at an intermediate stage, still prepare intermediates as optically inactive racemates. Accordingly, optical resolution, which can be expensive and time consuming, is still required at some point. Moreover, such optical resolutions can be best give a 50% yield of diltiazem.